Add sphinxcontrib-httpdomain

This is already used by Ceilometer from tox.ini actually, so let's add it
here.

Change-Id: Ib041b1b86b2f7d62e832aa979100fa7864d283b0
Signed-off-by: Julien Danjou <julien@danjou.info>
Merge "Add sphinxcontrib-httpdomain"